This invention relates to ceramic and quartz metal halide lamps and more particularly to such lamps having a red emission larger than the one of tungsten halogen sources of the same color temperature. Furthermore, the red color index R9 of these lamps exceeds by a substantial amount the R9 index of the conventional metal halide lamps. Lamps with enhanced red emission and improved color rendering are highly desirable in color critical applications.
The color rendering properties of lamps are expressed in terms of a single index, Ra. This index can be accompanied by 14 special indices which represent the color rendering properties of the specific test colors from CIE Publication 13.2 (1974). The R9 index is the red color rendering index for strong red with Munsell notation 4.5 R 4/13. One of the important advances that can be made toward the rendering of colors of metal halide lamps is to improve the red color rendering. Quartz metal halide lamps with a sodium-scandium-lithium chemistry have in general an Ra of about 75 and an R9 of about xe2x88x9265. Ceramic metal halide lamps with a sodium-rare earth chemistry can have a general index Ra greater than 85 and a special index R9 less than xe2x88x9215. With reference to a blackbody source of the same color temperature, the radiation of quartz and ceramic metal halide lamps in the red region of the spectrum is much lower. A blackbody source has a peak radiation in the infrared region and has much better red color rendering. The R9 index of a blackbody source is 100 (see FIG. 4 for nomenclature).
To improve the red color rendering, conventional metal halide lamps in color critical applications such as clothing retailing are combined with discharge lamps having excessive red radiation. The lamps with enhanced red emission can be of the type of white high pressure sodium lamps. Their amount of red exceeds the red radiation of the corresponding blackbody source of the same color temperature. This combination of the two different types of lamps is expensive and requires additional space for mounting.
W. Thornton (U.S. Pat. No. 4,029,983; 1977) introduced a metal halide lamp having a light output with incandescent characteristics. The quartz metal halide lamp employs a sodium-scandium discharge and a luminescent coating on the inner surface of the outer envelope. The coating comprises a blend of green emitting CaS:Ce phosphor and a red emitting CaS:Eu phosphor. The color temperature of the sodium-scandium arc discharge is 3500K to 3900K. E. F. Wyner (Journal of IES/July 1984) also improved the red radiation of quartz metal halide lamps by means of a phosphor coating. The red emission was achieved with yttrium vanadate and magnesium fluorogermanate phosphor coatings. The phosphors are activated by the ultraviolet radiation of the lamp. The color temperature averaged about 3000K through the life of the lamp. Caruso et al. (U.S. Pat. No. 4,742,268; 1988) invented a high color rendering calcium-containing metal halide lamp. The long-arc ellipsoidal arc tube provided a high cold spot temperature and an exceptional color rendition. Kramer et al. (U.S. Pat. No. 4,801,846; 1989) invented a rare earth halide light source with enhanced red emission. High efficacy, good color rendering, and a warm color temperature were attained by utilizing rare earth fills in conjunction with calcium halides and, or sodium halides. Ramaiah et al (U.S. Pat. No. 5,225,738; 1993) disclosed a metal halide lamp with improved lumen output and color rendering. The color temperature of the lamp containing sodium and scandium iodide was decreased to below about 3635K by the addition of critical amounts of thallium iodide and lithium iodide. The general Ra index was greater than about 75, however, the special rendering index R9 had a low negative value of xe2x88x9265 resulting in poor red color rendering. Krasko et al. (U.S. Pat. No. 5,694,002; 1997) described a new metal halide lamp with improved color characteristics. The CRI improved from 75 to 85 and the special index R9 increased from xe2x88x9265 to xe2x88x9215. The fill composition included the halides of sodium, scandium, lithium, dysprosium and thallium.
The present invention utilizes an unique construction to enhance the red emission and to improve the red color rendering. For a quartz or ceramic metal halide lamp, the radiant energy in the yellow region of the spectrum is reduced or filtered out at about 589 nm. In addition, the radiant energy in the red region is enhanced by increasing the salt vapor pressure and consequently by broadening the sodium D-line. The lamps with such a construction have enhanced red emission, well beyond the red amount in a blackbody source of the same color temperature. Furthermore, the special index R9 increases from xe2x88x9220 to about 55 (see FIG. 4).
The increase in the salt vapor pressure is most conveniently obtained by using heat shields made out of metal as described by Zhu et al. (Application Ser. No. 09/074623) and assigned to the same assignee as the present application. Alternatively, increased vapor pressure of the salts can be accomplished by increased wall loading. For ceramic arc tubes, one can safely go to about 3OW/cm2, while for quartz arc tubes, one probably should not exceed about 24W/cm2 so maintenance and life are not adversely affected. The increased vapor pressure leads to broadening of the Na xe2x80x9cDxe2x80x9d line which enhances the red and green emissions while the yellow around 590 nm is self-reversed somewhat. Subsequently, this radiation is subjected to a filter which filters out further the yellow around 590 mn resulting in purely enhanced red radiation. This leads to a somewhat reduced efficacy due to the removal of the yellow radiation. The particular characteristics of the broadening of the xe2x80x9cDxe2x80x9d lines and the filtering will be explained hereinafter.